My present invention pertains generally to the field of Raman spectroscopy. More particularly, the invention relates to a novel filter spectrograph having a high order of laser light rejection and providing a Raman spectrum display suitable of rapid analysis.
Basically, the Raman effect is an inelastic scattering process. If monochromatic light (such as from a well filtered laser beam) impinges on any material, the resultant signal can consist of elastically scattered incident light (Raleigh or Mie scattering), fluorescence (from inelastic absorption), and Raman lines whose frequency shift from the incident light is characteristic of the material. These frequency shifts correspond to the vibrational and rotational frequencies of the molecules and ions constituting the material. Since no two molecules have exactly the same vibrational frequencies, the Raman effect can be used as a "fingerprint" technique to identify molecular species.
It is well known that the chemical composition of gases, liquids and solids, the latter including relatively minute, Raman-active, single crystal particles, can be accurately determined from the spectra provided by a conventional Raman spectrometer system. In fact, measurements of Raman spectra of single crystal particles greater than 100 microns equivalent spherical diameter are presently being done routinely. One form of Raman spectrometer is, for example, shown, described and claimed in the U.S. Pat. No. 3,414,354 of Edouard H. Siegler, Jr. for Raman Spectrometers patented Dec. 3, 1968. There exists a need, however, to extend routine Raman analysis capabilities to turbid liquids and gases, and to particles from 5 microns to as small as 0.1 micron in size. Particles of interest include, for example, metal oxides, metal halides, heavy metal salts and hydrocarbon compounds from environmental sources.
A feasible micro-Raman spectrometer system, for example, for analyzing the very small single particles can consist of five subsystem elements including a focused laser source with controlled power density, a particle manipulator stage, a spectrographic light dispersing means for providing a Raman spectrum display, means for providing a parallel readout of the spectrum, and data reduction means for particle identification. In order to obtain a practical and useful micro-Raman spectrometer system, however, it was found that the spectral dispersing means must provide a laser line (Mie scattered light) rejection greater than 10.sup.11. This is ten times the smallest value of the Mie scattered laser signal to the strongest Raman signals and is necessary under certain predetermined system operating conditions to observe a Raman spectrum. Further, the total analysis time per particle must not exceed one hour in a suitable system. These requirements, among others, cannot be met with the conventional and commercially available rejection filters such as the Fabry-Perot interferometer and classical double monochromator.